Article pubs.acs.org/Macromolecules
Polymerization of Isobutylene by GaCl3 or FeCl3/Ether Complexes in Nonpolar Solvents Rajeev Kumar,† Philip Dimitrov,† Keith J. Bartelson,† Jack Emert,‡ and Rudolf Faust†,* †
Polymer Science Program, Department of Chemistry, University of Massachusetts Lowell, One University Avenue, Lowell, Massachusetts 01854, United States ‡ Infineum USA, 1900 East Linden Avenue, Linden, New Jersey 07036, United States S Supporting Information *
ABSTRACT: The carbocationic polymerization of isobutylene (IB), co-initiated by GaCl3 or FeCl3·dialkyl ether 1:1 complexes has been investigated in hexanes in the −20 to 10 °C temperature range. In contrast to AlCl3·diisopropyl ether (AlCl3·iPr2O) complexes,1 GaCl3·i-Pr2O and FeCl3·i-Pr2O readily co-initiate polymerization with 2-chloro-2,4,4-trimethylpentane (TMPCl) or tert-butyl chloride (t-BuCl) in the presence or absence of proton trap. In the absence of proton trap, chain transfer to monomer readily proceeded, resulting in close to complete monomer conversion and up to 85% exo-olefinic end group content. Diisopropyl ether complexes gave the highest polymerization rates, while nonbranched alkyl ether complexes were completely inactive. A polymerization mechanism is proposed to involve ether-assisted proton elimination to yield PIB exo-olefin, and the abstracted proton can subsequently start a new polymer chain by protonation of IB. Alternatively PIB+ may be deactivated by ion collapse to yield PIBCl, which can be reactivated by the Lewis acid. The reasons for the difference in behavior between the Ga and Fe catalysts and the Al-based catalysts are described.
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INTRODUCTION Low molecular weight (Mn ∼ 500−5000 g/mol) olefin end functional PIB is a precursor to motor oil and fuel additives. Currently two major industrial methods are utilized to produce low molecular weight IB homo or copolymers with olefinic end groups. The “conventional” method uses a C4 mixture and AlCl3 or EtAlCl2 based catalyst systems, which provides polybutenes with high trisubstituted olefinic content.2,3 The other method employs pure IB and uses BF3 complexes with either alcohols or ethers as catalysts, yielding highly reactive PIB (HR PIB) with high exo-olefinic end-group content.4 In contrast to the trisubstituted olefins of conventional polybutenes, PIB exo olefins readily react with maleic anhydride in a thermal ene reaction to produce PIB succinic anhydride and subsequently polyisobutenyl succinimide ashless dispersants. Since chlorination is not necessary for maleation of HR PIB, the final product does not contain any chlorine, making HR PIB more desirable than conventional polybutenes. In recent decades, several new methods for the synthesis of HR PIB have been reported. For example, PIBCl was selectively dehydrochlorinated by a bulky base, e.g., potassium tertbutoxide to yield HR PIB.5 Storey et al. used living cationic polymerization of IB at −80 °C to obtain living PIB, which was then end-quenched with sterically hindered bases6 or sulfides.7 Another method used to produce HR PIB, developed by Kuhn and co-workers, involves inorganic/organometallic catalysts with weakly coordinating anions in dichloromethane (DCM).8 Recently, Kostjuk and Wu independently reported that at moderate temperatures AlCl3·dibutyl ether9 (AlCl3·Bu2O) and AlCl3·i-Pr2O10 complexes in DCM or DCM/hexanes 80/20 (v/ v) mixtures give HR PIB with exo-olefinic end-groups in excess © XXXX American Chemical Society
of 90%. Shortly thereafter, Wu and co-workers also reported the use of FeCl3·i-Pr2O complexes for the polymerization of IB to yield HR PIB in DCM.11 Most recently, Kostjuk and coworkers demonstrated that the AlCl3·Bu2O system could be used in conjunction with cumyl alcohol (CumOH) or water as an initiator in nonpolar solvents, such as toluene or hexane, to produce HR PIB with exo-olefin content in excess of 85%.12 Our recent reexamination of the polymerization of IB with the CumOH/AlCl3·Bu2O initiator/co-initiator system in DCM/hexanes (80/20 v/v) at −40 °C, previously reported by Kostjuk and co-workers,9 revealed that CumOH is not an initiator in conjunction with AlCl3·Bu2O and that the true initiator is adventitious water.1 Similarly, in 100% hexanes at 0 °C, none of the conventional cationic initiators such as cumyl chloride (CumCl) or TMPCl could initiate polymerization of IB. The polymerization of IB could only be initiated with water, but monomer conversions and exo-olefin content (60−70%) were lower and polydispersity indices (PDIs) were much higher than in polar solvents.9 Mechanistic studies suggested that the reaction of water with AlCl3·R2O yields H+AlCl3OH− that initiates polymerization and releases free ether, which subsequently abstracts a β-proton from the growing chain end before it diffuses from the immediate vicinity of the polymer cation.12 Since the AlCl3·R2O complex yields PIB exoolefins with high selectivity only in polar solvent and only water initiates the polymerization, we have studied other Lewis acid/ Lewis base complexes that could potentially initiate polymerReceived: August 21, 2012 Revised: September 26, 2012
A
dx.doi.org/10.1021/ma3017585 | Macromolecules XXXX, XXX, XXX−XXX
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ization of IB in conjunction with conventional cationic initiators in nonpolar solvents. The present study describes the polymerization of IB initiated with tertiary alkyl chlorides in conjunction with GaCl3 or FeCl3·ether complexes in hexanes. Under these conditions, high exo-olefinic end-group HR PIB with molecular weights ranging from 1000 to 3000 g/mol and PDIs of ∼2 were obtained at high monomer conversion.
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EXPERIMENTAL SECTION
Materials. For the experiments with GaCl3, hexanes and dichloromethane (DCM) were purified as described previously.13 For experiments with FeCl3, hexanes and DCM were either purified as described previously13 or their anhydrous grades (Aldrich) were used without purification. Isobutylene (IB, Matheson Tri Gas) was dried in the gaseous state by passing it through in-line gas-purifier columns packed with BaO/Drierite and then condensed in a receiver flask at −30 °C before use. Gallium trichloride (GaCl3, 99.99%, Aldrich), iron trichloride (FeCl3, 97%, Aldrich), 2-chloro-2-methylpropane (t-BuCl, >98%, TCI America), cumyl alcohol (CumOH, 97%, Aldrich), tertbutyl alcohol (t-BuOH, 99.6%, anhydrous, Aldrich), dibutylether (Bu2O, 99.3%, anhydrous, Aldrich), diisopropyl ether (i-Pr2O, 99%, anhydrous, Aldrich), di-sec-butyl ether (sec-Bu2O, 96%, Aldrich), diisobutyl ether (i-Bu2O, 99%, TCI America), butyl methyl ether (BuOMe, 99%, Aldrich), tert-butyl methyl ether (t-BuOMe, 99.8%, Aldrich), trifluoroacetic acid (TFA, 99+%, Aldrich), and 2,6-di-tertbutylpyridine (DTBP, 97%, Aldrich) were used as received. 2-Chloro2,4,4-trimethylpentane (TMPCl) was synthesized according to a literature procedure.14 Preparation of GaCl3 or FeCl3·Dialkyl Ether Complexes. GaCl3 or FeCl3·dialkyl ether complexes were prepared right before the polymerization of IB. In the glovebox, DCM was added to the Lewis acid (GaCl3 or FeCl3), which had been previously weighed and sealed in a 20 mL vial with a Teflon septum. The GaCl3 quickly dissolved whereas the FeCl3 was mostly insoluble. Next, an equimolar amount of the appropriate ether was added slowly via a syringe to the sealed vial containing the Lewis acid while stirring to form a 1.0 M Lewis acid/ ether complex solution. Polymerization of IB. Polymerizations were performed under a dry N2 atmosphere in an MBraun 150-M glovebox (Innovative Technology Inc., Newburyport, MA). IB was condensed and distributed to the polymerization reactors, screw top culture tubes (75 mL), at −30 °C. The polymerizations, which were co-initiated with GaCl3 or FeCl3/ether complexes (typically 0.02 M) at monomer concentrations of [IB] = 1.0 M, were performed in hexanes at temperatures ranging from −20 to +10 °C, and terminated with either NH4OH or MeOH. Characterization. Size Exclusion Chromatography. Molecular weights and polydispersities were obtained from size exclusion chromatography (SEC) with universal calibration using a Waters 717 Plus autosampler, a 515 HPLC pump, a 2410 differential refractometer, a 2487 UV−vis detector, a MiniDawn multi angle laser light scattering (MALLS) detector (measurement angles are 44.7°, 90.0°, and 135.4°) from Wyatt Technology Inc., a ViscoStar viscosity detector from Wyatt Technologies Inc., and five Ultrastyragel GPC columns connected in the following order: 500, 103, 104, 105, and 100 Å. The RI was the concentration detector. Tetrahydrofuran was used as the eluent at a flow rate of 1.0 mL/min at room temperature. The results were processed using the Astra 5.4 software from Wyatt Technology Inc. Nuclear Magnetic Resonance. Proton nuclear magnetic resonance (1H NMR) spectra were recorded on a Bruker 500 MHz spectrometer using CDCl3 or CD2Cl2, as solvents (Cambridge Isotope Lab., Inc.). The PIB end group content was determined by 1H NMR spectroscopy (Figure 1). The two protons characteristic of the exo-olefin end group (structure A, protons a1 and a2) are well resolved and appear at 4.85 and 4.64 ppm, while the one proton characteristic of the endo-olefin end group (structure B, proton d) appears at 5.15 ppm. Small amounts of the E and Z configurations of another trisubstituted olefin end group (Structure C, protons e1 and e2) could also be detected in some
Figure 1. Typical 1H NMR spectrum of HR PIB obtained in this study. samples at 5.37 and 5.17 ppm. The tetra-substituted olefin end group (structure D, proton f) appears as a broad multiplet at 2.85 ppm. Resonances for coupled PIB chains (structure E, protons g) are normally found at 4.82 ppm. The methylene protons of the PIBCl end group (structure F, protons h) at 1.96 ppm were used to determine the content of PIBCl. Finally, the methylene and methyl protons of the IB repeat unit (structure A, protons b and c, respectively) could be observed at 1.42 and 1.11 ppm, respectively. Fourier Transform Infrared Spectroscopy. Fourier transform infrared spectroscopy (FT-IR) was performed in situ using a Mettler Toledo ReactIR 4000 instrument equipped with a DiComp probe connected to an MCT detector with a K6 conduit. Sampling wavenumbers were from 4000 to 650 cm−1 at a resolution of 2 cm−1.
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RESULTS AND DISCUSSION Complexes of gallium trichloride (GaCl3) or iron trichloride (FeCl3) with various branched and nonbranched alkyl ethers were screened for their potential as co-initiators for the polymerization of IB in conjunction with t-BuCl or TMPCl (Table 1). In all cases where polymerization was observed, PIB with high exo-olefinic end-group content (71−82%) was obtained. Steric effects seem to be crucial for the polymerization outcome as both GaCl3 and FeCl3 form stable complexes with sterically unhindered ethers such as diethyl ether (Et2O), butyl methyl ether (BuOMe), and dibutyl ether (Bu2O), but initiation and polymerization of IB were absent. The best results with respect to monomer conversions and exoolefin content were obtained with the more sterically hindered i-Pr2O. Polymerizations of IB initiated by TMPCl or t-BuCl and coinitiated by GaCl3·i-Pr2O or FeCl3·i-Pr2O complexes in the presence or absence of the proton trap 2,6-di-tert-butylpyridine (DTBP) are summarized in Table 2. In contrast to AlCl3·iPr2O, which could not ionize TMPCl to initiate polymerization of IB, initiation proceeds readily with either GaCl3·i-Pr2O or FeCl3·i-Pr2O (entries 4 and 11, Table 2). With the GaCl3·iPr2O complex in the presence of DTBP at [TMPCl] = 0.01 M, 16% conversion was reached in 20 min and the concentration of PIB was practically equal to the concentration of TMPCl (Entry 3, Table 2). A 2-fold increase in [TMPCl] almost doubled the conversion; however, the concentration of PIB increased almost 3 fold, since the low concentration of DTBP could not completely suppress chain transfer to monomer (entry 3 vs 4, Table 2). Importantly, PIB with high exo-olefin B
dx.doi.org/10.1021/ma3017585 | Macromolecules XXXX, XXX, XXX−XXX
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Table 1. Polymerization of IB in Hexanes at 0°C for 20 min Initiated by t-BuCl and Coinitiated by Lewis Acid·Ether Complexesa entry
Complex (LA·ether)
convnb (%)
Mn (g/mol)
PDI
Mn,NMRc (g/mol)
[PIB]d (M)
exoe (%)
tri + endoe (%)
tetrae (%)
PIBCle (%)
couplede (%)
1f 2 3 4 5 6 7 8 9
GaCl3·i-Pr2O GaCl3·sec-Bu2O GaCl3·i-Bu2O GaCl3·Bu2O GaCl3·BuOMe GaCl3·Et2O FeCl3·i-Pr2O FeCl3·BuOMe FeCl3·Et2O
100 74 30 0 0 0 90 0 0
900 2000 2000 − − − 1200 − −
2.1 2.0 1.8 − − − 2.2 − −
1000 1800 1600 − − − 950 − −
0.056 0.023 0.011 − − − 0.054 − −
77 71 74 − − − 81 − −
9 10 10 − − − 9 − −
8 10 9 − − − 8 − −
7 9 7 − − − 0 − −
0 0 0 − − − 2 − −
a [IB] = 1.0 M, [t-BuCl] = 0.02 M, and [LA·ether] = 0.02 M; terminated with MeOH. bGravimetric conversion. cMn, NMR= 56.11 × [(b/2)/((a1 + a2)/2) + d + e1 + e2 + (g/2) + (h/2)]. See Figure 1 for proton assignments. d[PIB]=[IB] x 56.11 × (convn/Mn,NMR). eDetermined by 1H NMR spectroscopy. fTMPCl was used instead of t-BuCl.
Table 2. Polymerization of IB in Hexanes at 0°C for 20 min Initiated by TMPCl or t-BuCl and Coinitiated by LA·i-Pr2Oa entry
LA
TMPCl (M)
t-BuCl (M)
DTBP (M)
convnb (%)
Mn (g/mol)
PDI
Mn ,NMRc (g/mol)
PIBd (M)
exoe (%)
tri + endoe (%)
tetrae (%)
PIBCle (%)
couplede (%)
1 2 3 4 5 6 7f 8g 9f 10g 11f 12f 13f 14g 15f
GaCl3 GaCl3 GaCl3 GaCl3 GaCl3 GaCl3 FeCl3 FeCl3 FeCl3 FeCl3 FeCl3 FeCl3 FeCl3 FeCl3 FeCl3
0 0.02 0.01 0.02 0 − 0 0 0.02 0.02 0.02 0 − − −
− − − − − 0.01 − − − − − − 0.02 0.02 0.02
0 0 0.005 0.005 0.005 0 0 0 0 0 0.005 0.005 0 0 0.005
26 98 16 30 2 57 41 39 93 98 42 0 90 84 21
1700 1400 1300 1000 − 1700 2200 2100 1000 900 800 − 1200 1000 900
1.9 1.4 2.2 1.7 − 1.5 4.5 4.0 2.2 2.4 2.1 − 2.2 2.6 2.1
1500 1000 1000 700 1000 1400 2100 2000 800 800 700 − 1000 800 900
0.010 0.055 0.009 0.024 0.001 0.023 0.011 0.011 0.065 0.069 0.034 − 0.050 0.059 0.013
68 71 68 67 93 70 75 72 82 81 87 − 81 77 85
16 11 8 6 0 8 11 13 12 7 6 − 9 8 8
10 10 7 5 0 6 12 12 6 8 4 − 8 7 7
5 7 17 22 7 16 0 0 0 3 0 − 0 4 0
0 0 0 0 0 0 2 3 0 1 3 − 2 4 0
[IB] = 1.0 M, [Initiator] = 0, 0.01, or 0.02 M, and [LA·ether] = 0.02 M; terminated with MeOH. bGravimetric conversion. cMn,NMR= 56.11 × [(b/ 2)/((a1 + a2)/2) + d + e1 + e2 + (g/2) + (h/2)] See Figure 1 for proton assignments. d[PIB]=[IB] x 56.11 × (convn/Mn,NMR). eDetermined by 1H NMR spectroscopy. fExperiment performed with freshly distilled DCM and hexanes prepared as described in the Experimental Section. gExperiment performed with anhydrous DCM and hexanes without further purification. a
Scheme 1. Possible Mechanism for the Polymerization of IB by TMPCl and LA·i-Pr2O
with the GaCl3·i-Pr2O system compared to the FeCl3·i-Pr2O system. For [TMPCl] = 0.02 M in the absence of a proton trap, near quantitative conversion was observed using either GaCl3·i-Pr2O or FeCl3·i-Pr2O, and the exo-olefin content was high in both cases (71% and 82%, respectively). All PIBs exhibited monomodal molecular weight distributions and acceptably
content was obtained regardless of initiator concentration. In addition to a small amount of endo- and tetra-substituted olefins, a small amount of PIBCl chain ends was also observed with the GaCl3·i-Pr2O complex but not with the FeCl3·i-Pr2O complex (entry 4 vs entry 11, Table 2). Apparently, deactivation by ion collapse is faster (and ionization is slower) C
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Table 3. Evolution of Polymerization of IB with Time in Hexanes at 0 °C Initiated by TMPCl and Coinitiated by LA·i-Pr2Oa entry
LA
time (min)
convnb (%)
Mn (g/mol)
PDI
Mn ,NMRc (g/mol)
[PIB]d (M)
exoe (%)
tri + endoe (%)
tetrae (%)
PIBCle (%)
couplede (%)
1 2 3 4 5 6 7 8 9 10
GaCl3 GaCl3 GaCl3 GaCl3 GaCl3 FeCl3 FeCl3 FeCl3 FeCl3 FeCl3
5 10 20 40 60 5 10 20 40 60
38 52 64 77 84 48 57 71 86 86
1400 1400 1800 1700 1400 1400 1300 1200 1200 1400
2.2 1.9 1.7 1.6 1.8 2.3 2.5 2.4 2.3 2.3
1200 1300 1400 1400 1300 1100 1200 1100 900 800
0.018 0.022 0.026 0.031 0.036 0.024 0.027 0.036 0.054 0.060
54 74 79 80 72 80 79 82 83 83
7 10 8 11 14 7 6 7 7 8
5 6 6 7 10 3 6 7 8 5
34 10 7 2 3 6 6 3 1 0
0 0 0 0 0 3 2 1 1 3
a [IB] = 1.0 M, [TMPCl] = 0.01 M, and [LA·i-Pr2O] = 0.02 M; terminated with MeOH. bGravimetric conversion. cMn,NMR= 56.11 × [(b/2)/((a1 + a2)/2) + d + e1 + e2 + (g/2) + (h/2)]. See Figure 1 for proton assignments. d[PIB]=[IB] × 56.11 × (convn/Mn,NMR). eDetermined by 1H NMR spectroscopy.
Table 4. Polymerization of IB in Hexanes at 0 °C for 20 min Initiated by TMPCl and Coinitiated by LA·i-Pr2O.a entry
LA
[TMPCl] (M)
[LA·i-Pr2O] (M)
Conv.a (%)
Mn (g/mol)
PDI
Mn ,NMRb (g/mol)
[PIB]c (M)
exod (%)
tri+endod (%)
tetrad (%)
PIBCld (%)
Coupledd (%)
1 2 3 4 5 6 7 8
GaCl3 GaCl3 GaCl3 GaCl3 FeCl3 FeCl3 FeCl3 FeCl3
0.005 0.010 0.020 0.020 0.005 0.010 0.020 0.020
0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.02
20 34 59 98 31 47 60 98
1500 1300 1300 1400 1200 1200 1000 900
1.8 1.9 1.4 1.4 2.8 2.6 2.2 2.4
1300 1200 900 1000 1100 900 800 800
0.009 0.016 0.037 0.055 0.016 0.029 0.043 0.070
61 59 57 71 83 82 83 81
8 7 7 11 8 9 8 7
6 5 4 10 3 8 7 8
25 29 32 7 0 0 0 3
0 0 0 0 1 2 2 1
Gravimetric conversion. bMn,NMR = 56.11 × [(b/2)/((a1 + a2)/2) + d + e1 + e2 + (g/2) + (h/2)] See Figure 1 for proton assignments. c[PIB]=[IB] x 56.11 × (convn/Mn,NMR). dDetermined by 1H NMR spectroscopy. e[IB] = 1.0 M; terminated with MeOH.
a
low PDIs of ∼2, which is much lower than what was previously observed for AlCl3·ether systems in hexanes.1 The polymerization of IB by either GaCl3·i-Pr2O or FeCl3·i-Pr2O in the absence of any added initiator resulted in low conversions (entries 1, 7, and 8, Table 2). Presumably the real initiator in these cases was adventitious water, as suggested by the negligible conversions obtained in the presence of the proton trap DTBP (entries 5 and 12, Table 2). The lower conversions in the presence of DTBP for both complexes can be attributed to the suppression of protic initiation from adventitious water as well as the suppression of chain transfer by terminative proton entrapment. The molecular weight obtained with the GaCl3 complex in the presence of DTBP (entry 4 in Table 2) is also lower compared to that observed in the absence of DTBP (entry 2 in Table 2). This suggests that deactivation is faster than proton transfer (see Scheme 1) and a growing polymer chain may undergo many activation−deactivation cycles before proton elimination. Furthermore, there is a higher concentration of lower molecular weight PIBCl (22 mol % in entry 4 in Table 2) in the presence of DTBP, thus the overall molecular weight of the polymer is lower. As with the AlCl3/ether systems, when GaCl3 and i-Pr2O were added separately, low exo-olefinic end-group content was observed due to the reduced local concentration of ether, which is needed to mediate the ß-proton elimination from PIB+ (Table S1, Supporting Information). Interestingly, the GaCl3 system was much more sensitive to solvent purity (especially that of DCM) than the FeCl3 system. If DCM was not rigorously purified, conversions and/or exo content decreased. However, with the FeCl3·i-Pr2O complex, essentially identical results were obtained in highly purified solvents or anhydrous
grade solvents without purification (entries 7−15, Table 2). Thus, we elected to perform the remaining FeCl3·i-Pr2O complex studies with commercially available anhydrous solvents. A plausible mechanism for the polymerization of IB by TMPCl and LA·i-Pr2O is presented in Scheme 1. TMPCl is ionized by LA·ether complex to give TMP+, which initiates the IB polymerization, and this ultimately results in the release of ether from the complex. The propagating PIB+ may undergo proton elimination to yield PIB exo olefin or it may be deactivated by ion collapse to yield PIBCl, which can be reactivated by the Lewis acid. The abstracted proton may then start a new polymer chain by cationation of IB. Polymerizations with both GaCl3·i-Pr2O and FeCl3·i-Pr2O at 0 °C in hexanes proved to be quite rapid, reaching near complete conversion after 20 min. Quenching the polymerization at different time intervals, revealed the progression of the reaction (Table 3). In the case of GaCl3·i-Pr2O, monomer conversions increased and PIBCl content decreased with time due to reionization followed by ß-proton elimination (entries 1−5, Table 3). On the other hand, FeCl3·i-Pr2O coinitiated the polymerization at a higher rate and the PIBCl content was much lower at all stages of the polymerization. This suggests that FeCl3·i-Pr2O is a stronger Lewis acid than GaCl3·i-Pr2O (entries 6−10, Table 3). Thus, significantly higher exo-olefin content was also obtained with the FeCl3·i-Pr2O complex at the early stages of the polymerization. The effect of initiator and co-initiator concentrations on conversion, molecular weight, and PIBCl content with GaCl3·iPr2O and FeCl3·i-Pr2O as coinitiators is presented in Table 4. As expected, monomer conversions increased with increasing D
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Table 5. Polymerization of IB in Hexanes for 20 min Initiated by TMPCl and Coinitiated by LA·i-Pr2Oa entry
LA
temp (°C)
convnb (%)
Mn (g/mol)
PDI
Mn NMRc (g/mol)
[PIB]d (M)
exoe (%)
tri + endoe (%)
tetrae (%)
PIBCle (%)
couplede (%)
1 2 3 4 5 6 7 8
GaCl3 GaCl3 GaCl3 GaCl3 FeCl3 FeCl3 FeCl3 FeCl3
−20 −10 0 10 −20 −10 0 10
81 80 100 100 21 53 98 100
1400 1100 900 1000 900 1000 900 1200
2.6 2.3 2.1 2.1 3.5 2.6 2.4 2.4
1400 1200 1000 700 500 900 800 700
0.032 0.037 0.056 0.080 0.024 0.033 0.070 0.080
46 67 77 75 30 71 81 79
15 5 9 11 8 4 7 10
6 3 8 12 2 5 8 11
34 24 7 2 57 18 3 0
0 0 0 0 3 3 1 0
[IB] = 1.0 M, [TMPCl] = 0.02 M, and [LA·i-Pr2O] = 0.02 M; terminated with MeOH. bGravimetric conversion. cMn,NMR= 56.11 × [(b/2)/((a1 + a2)/2) + d + e1 + e2 + (g/2) + (h/2)]. See Figure 1 for proton assignments. d[PIB]=[IB] x 56.11 × (convn/Mn,NMR). eDetermined by 1H NMR spectroscopy. a
concentration of TMPCl or LA·i-Pr2O, while the molecular weights decreased slightly. At low LA·i-Pr2O concentrations, initiator efficiency decreased and PIBCl content increased slightly with increasing [TMPCl]. The PIBCl content decreased with increasing LA·i-Pr2O concentration, as there are more ionization cycles for the given polymerization time (20 min). It is also noteworthy that the FeCl3 system generally provided higher exo-olefin content and lower PIBCl content than the GaCl3 system. To study the effect of polymerization temperature, the temperature was varied from −20 to 10 °C (Table 5). For the GaCl3·i-Pr2O system, the molecular weight increased slightly with decreasing temperature as expected (entries 1−4, Table 5). However, this was not true with the FeCl3·i-Pr2O system as a substantial amount of PIBCl was observed below −10 °C (entries 5−8, Table 5). This could be due to the fact that reionization of PIBCl with FeCl3 is more temperature dependent than with GaCl3. In the case of the GaCl3·i-Pr2O system, the PDIs were generally ∼2.0 regardless of the temperature. However, in the case of the FeCl3·i-Pr2O system, the PDIs tended to be slightly higher at ∼2.4 and increased with decreasing temperature. The exo-olefin end-group content increased with increasing temperature up to 0 °C (due to higher tendency of proton elimination at higher temperatures). However, at temperatures above 0 °C, the exo-olefin end-group content decreased slightly due to increased carbocationic rearrangements. For both systems, the observed increase of exo-olefinic PIB with increasing temperature came at the expense of PIBCl, which decreased with increasing temperatures (Figure 2). Polymerization of IB in the presence of only 0.003 M uncomplexed GaCl3 and 0.02 M t-BuCl resulted in complete monomer conversion and produced only conventional PIB with no exo-olefin end groups (entry 1, Table 6). This suggests that to maximize exo-olefin with GaCl3·i-Pr2O, it is critical to achieve an exact 1:1 stoichiometry between the Lewis acid and the ether with no free GaCl3. However, when free FeCl3 was employed in the polymerization in a similar manner as free GaCl3, no polymer was obtained (entry 3, Table 6). This can be attributed to the insolubility of free uncomplexed FeCl3 in the polymerization system, while free GaCl3 is soluble. Because of this difference in solubility, exo-olefin can be obtained even when [FeCl3] > [i-Pr2O] though the amount is reduced somewhat. On the other hand, too much ether reduces monomer conversion. Prompted by these results, we performed in situ FT-IR studies on the formation of the LA·i-Pr2O complexes with
Figure 2. 1H NMR spectra overlay for products of polymerization of IB by [GaCl3·i-Pr2O] = 0.02 M and [TMPCl] = 0.02 M in hexanes for 20 min at different temperatures; terminated with NH4OH.
GaCl3 and FeCl3. Similar spectroscopic studies have been performed by Wu with FeCl3·i-Pr2O complexes and our findings agree with what was reported, which is that FeCl3 and ether form a 1:1 complex. 11 Our FT-IR studies with the GaCl3·i-Pr2O complex exhibit the same phenomenon (Figure 3). With excess amounts of GaCl3, the characteristic C−O stretch of the diisopropyl ether (1011 cm−1) is not observed, and this holds true up to a ratio of GaCl3:i-Pr2O = 1:1 (Figure 3, red and blue spectra). However, once excess ether is introduced, free ether is readily observed in the spectrum, suggesting 1:1 complex formation between GaCl3 and diisopropyl ether (Figure 3, green spectrum). Polymerizations of IB by LA·i-Pr2O complexes were also attempted with CumOH, tBuOH, TFA, and t-BuOMe as initiators. Polymerization was observed only for CumOH and tBuOH. However, monomer conversions were similar or lower than those performed in the absence of any initiator. This suggests that similarly to AlCl3·dialkyl ether complexes,1 GaCl3 or FeCl3·dialkyl ether complexes are also unable to ionize these alcohols.
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CONCLUSION In contrast to AlCl3·dialkyl ether complexes, GaCl3 or FeCl3·branched dialkyl ether complexes readily coinitiate the polymerization of IB in conjunction with alkyl halides such as E
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Table 6. Polymerization of IB in Hexanes at 0 °C for 20 min Initiated by t-BuCl and Coinitiated by LA or FeCl3·i-Pr2Oa entry 1 2 3 4 5 6 7 8 9
coinitiator, (M)
LA: iPr2O
[t-BuCl] (M)
convnb (%)
Mn (g/mol)
PDI
Mn ,NMRc (g/mol)
[PIB]d (M)
exoe (%)
tri + endoe (%)
tetrae (%)
PIBCle (%)
couplede (%)
GaCl3, 0.003 GaCl3, 0.003 FeCl3, 0.004 FeCl3, 0.004 FeCl3·i-Pr2O, 0.02 FeCl3·i-Pr2O, 0.02 FeCl3·i-Pr2O, 0.02 FeCl3·i-Pr2O, 0.02 FeCl3·i-Pr2O, 0.02
1:0 1:0 1:0 1:0 1:0.90
0.02 0 0.02 0 0.02
100 100 0 0 93
− − − − 1000
− − − − 2.9
700 1500 − − 800
0.080 0.037 − − 0.064
0 0 − − 67
53 67 − − 17
47 33 − − 14
0 0 − − 0
0 0 − − 2
1:0.95
0.02
99
1000
3.1
900
0.062
72
13
13
0
3
1:1
0.02
84
1000
2.6
800
0.059
77
8
7
4
4
1:1.05
0.02
81
1100
2.7
900
0.050
79
7
7
5
2
1:1.10
0.02
59
1200
2.6
900
0.037
78
6
6
7
3
[IB] = 1.0 M; terminated with MeOH. bGravimetric conversion. cMn,NMR= 56.11 × [(b/2)/((a1 + a2)/2) + d + e1 + e2 + (g/2) + (h/2)]. See Figure 1 for proton assignments. d[PIB]=[IB] x 56.11 × (convn/Mn,NMR). eDetermined by 1H NMR spectroscopy.
a
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ASSOCIATED CONTENT
S Supporting Information *
Table of polymerization data. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■ ■
ACKNOWLEDGMENTS Financial support from Infineum USA is greatly appreciated. REFERENCES
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Figure 3. In situ FT-IR study with GaCl3·i-Pr2O complexes in DCM at 25 °C. [GaCl3·i-Pr2O] = 0.35 M.
TMPCl or t-BuCl in nonpolar solvents such as hexanes at moderate temperatures, yielding PIB with up to 80% exoolefinic end groups. Linear dialkyl ether complexes are not active due to the lack of steric strain leading to increased stability of the complexes. Polymerization with FeCl3·ether complexes is generally faster than for the corresponding Ga complexes and the amount of PIBCl is lower in the product. This is attributed to faster activation and slower deactivation due to the stronger Lewis acidity of the FeCl3. The proposed mechanism, which involves displacement of the ether from the complex by the alkyl halide followed by ether-assisted deprotonation also explains why the AlCl3·dialkyl ether complexes are completely inactive with alkyl halides. In line with the findings of Kanazawa at al.,15 the activity of a Lewis acid/ether system for the ionization of an alkyl halide depends not only on the Lewis acidity of the metal halide, but also on the balance of its oxophilicity/chlorophilicity. The results suggest that AlCl3 is much more oxophilic than chlorophilic and that alkyl halides cannot displace the ether from the complex, while GaCl3 and FeCl3 are more chlorophilic. F
dx.doi.org/10.1021/ma3017585 | Macromolecules XXXX, XXX, XXX−XXX
